Electrical fires in machinery frequently arise from an electrical fault causing ignition and combustion of polymer materials such as the insulating cladding of wires and cables. Such fires are a serious problem presenting a potential hazard to both workers and property, not only from the fire but also from the evolution of toxic gases derived from thermal degradation of the polymer materials. The problem may be compounded when access to the point of ignition is physically restricted, e.g., within machines, circuitry etc., thereby limiting manual attempts to extinguish or even recognise the fire. Accordingly, there has been much interest in the development of polymer compositions having improved resistance to combustion.
It is known that blending certain compounds into the pre-polymer composition may have a flame retarding effect on the combustion of the final polymer products. Halogen-containing flame retardant additives are known and have been used to reduce the flammability of polymers such as polyvinyl chloride. However, there is an increasing awareness of the problems associated with halogenated flame retardants. These retardants are known to cause both high smoke generation and the emission of toxic and corrosive gases which may damage adjacent circuitry etc. These gases may also have wider reaching deleterious environmental effects. Alternatives to halogenated retardants can be broadly classed into either (a) phosphorus based retardants or (b) non-phosphorus based retardants.
Most non-phosphorus materials comprise inert filler materials such as calcium carbonate which have little effect. Some inorganic salts, e.g., alumina trihydrate, magnesium hydroxide and magnesium carbonate, have been used as flame retardant additives for polyolefin polymers. Unfortunately such salts require high loading by weight of the polymer composition to achieve the desired flammability performance. This high loading is found to have a negative effect on the physical properties of the polymer, e.g., tensile strength. Additionally salts such as alumina trihydrate have poor thermal stability.
The use of phosphorus based retardants is known, particularly the use of inorganic phosphates to generate intumescent formulations which cause the formation of a protective foamed char when exposed to heat thereby preventing further combustion. Such formulations have generally found applications in cellulosic type materials, resins and formulations containing low molecular weight polyols for paints and other such coating materials. Various combinations of phosphorus containing compounds are disclosed in, for example, Chemical Abstracts 92 111554 and 96 143939; U.S. Pat. Nos. 3810862, 4001177, 4140660, 4198493, 4341694, 4433115, 4491644 and 4742088; P.C.T. No. WO85/00220; British Patent Nos. 2142638 and 218114; Australian Patent Application No. 77342/87 and European Patent Nos. 0026391, 0045835 and 0115871. Such prior art retardants are found to suffer from one or more of the following disadvantages:
a) water sensitivity,
b) migration of the additive due to non-compatibility between the component compounds and the polymer matrix,
c) the deleterious action of the additive on the physical properties of the polymer, and
d) those retardants used successfully have a high specificity. For example, U.S. Pat. No. 4,491,644 discloses an additive comprising a blend of bis-melaminium pentate and a polyhedric oxide formulated for use principally in polypropylene. Such materials are commercially available under the trade name Charguard 329. Likewise, materials in accordance with European Patent No. 0115871, comprising a nitrogen containing oligomer and ammonium polyphosphate, commercially available under the trade name Spinflam MF82, are produced in grades specific to a particular polymer, e.g., polyethylene.
Thus, there is a need for a high performance method of flame retardation for use in polymers.